My players all have backgrounds. So far, only a couple of them have had family members show up in the game. The family of my female player's PC owns a bakery in the main city. The group has visited them a few times. I have had the PC's mom try to marry her off so she will stop this adventuring nonsense. Another PC had his family visited by the BBEG. Nothing bad happened, the BBEG went to visit, claiming he was a friend of the PC, and left a gift for the PC. It was lots of fun watching the PC trying to open the box without touching it, thinking it was trapped. In this case, it was not trapped but contained a couple of scrolls for the PC.
I have yet to need to put the PC's families in danger. It may happen in the future if they are in the wrong place at the wrong time.
As for other NPC's, some are one shots, some show up from time to time to offer information to the PCs or, in the case of merchants, items or adventure hooks. Right now, I have a shipping company in my campaign that the PCs have met the owner. The shipping company is having trouble with some of its ships getting attacked. Some of the PCs want to investigate as a favor, and the others because of the potential reward.